OH maser disc and outflow in the Orion-BN/KL region

Abstract

MERLIN measurements of 1.6-GHz OH masers associated with Orion-BN/KL are presented, and the data are compared with data on other masers, molecular lines, compact radio continuum sources and infrared sources in the region. OH masers are detected over an area 30 arcsec in diameter, with the majority lying along an approximately E-W structure that extends for approx. 18~arcsec, encompassing the infrared sources IRc2, IRc6 and IRc7. Radial velocities range from -13 to +42 km/s. The system of OH masers shows a velocity gradient together with non-circular motions. The kinematics are modelled in terms of an expanding and rotating disc or torus. The rotation axis is found to be in the same direction as the molecular outflow. There is an inner cavity of radius approx 1300 au with no OH masers. The inner cavity, like the water `shell' masers and SiO masers, is centred on radio source I. Some of the OH masers occur in velocity-coherent strings or arcs that are longer than 5~arcsec (2250 au). One such feature, Stream A, is a linear structure at position angle approx 45 degree, lying between IRc2 and BN. We suggest that these masers trace shock fronts, and have appeared, like a vapour trail, 200 yr after the passage of the runaway star BN. The radio proper motions of BN, source I and source n project back to a region near the base of Stream A that is largely devoid of OH masers. The 1612-MHz masers are kinematically distinct from the other OH masers. They are also more widely distributed and appear to be associated with the outflow as traced by water masers and by the 2.12-micron emission from shocked H2 The magnetic field traced by the OH masers ranges from 1.8 to 16.3 mG, with a possible reversal. No OH masers were found associated with even the most prominent proplyds within 10 arcsec of Theta1 Ori C.